Most of the supporting or carrying structural elements and assemblies in vehicles, cranes, aircraft and the like are not only statically loaded but are also often subject to dynamic loads. In that regard, damages can arise on the structural elements or assemblies not only upon exceeding a static limit load, but also due to a plurality of small dynamic loads, which lead to grain changes in the materials, whereby it can lead to damages due to material fatigue. Especially in the aircraft construction, dynamic loads often arise due to the cyclic loads in the various flight phases, whereby these dynamic loads cause such a material fatigue over a long time span. Therefore, aircraft are routinely inspected or examined for material fatigue damages, after prescribed flight hours, by ultrasonic examinations or x-raying. Such inspections or examinations are very time-consuming and expensive, and should therefore only occur when such damages can be counted on in accordance with expectations.
Therefore, it is attempted already during the development and construction phase, to determine by continuous load tests the critical locations on aircraft parts that can lead to such material fatigue manifestations. Through such continuous load tests it is therefore also possible, for each individual aircraft or its parts, to specify a number of flight hours, after which certain material fatigue tests or examinations shall be carried out, or in total to determine a maximum number of flight hours of an aircraft, after which a further operation is no longer justifiable for safety reasons due to material fatigue manifestations.
For such continuous load tests, in turn very complex or costly testing systems are necessary, through which partially the entire operating life of an aircraft must be simulated, detected and evaluated. For that purpose, up to 7000 strain gages are applied partially on the entire aircraft or on the critical parts thereof, such as fuselage or carrying surfaces, for example. In that regard, each individual strain gage is usually embodied as a quarter bridge and is supplemented with further supplemental resistances to form a Wheatstone measuring bridge, and is connected with an amplifier channel of an amplifier apparatus by a three to six wire measuring line or cable. By such an amplifier apparatus having up to 7000 channels, the individual measuring signals are amplified and digitized, and are then stored in a following memory and calculating or processing circuit, and are indicated or displayed or signaled as measured values. In a continuous load test, the aircraft to be inspected or its individual parts are now acted or impinged on by a usually hydraulic loading apparatus, with an alternating load that is modeled on the flight operation. In that regard, then both the introduced forces as well as the strains caused thereby on the aircraft parts are detected in time sequences and are stored for the evaluation. Then material fatigue manifestations can be determined from the corresponding strain progression or course on the critical aircraft parts after a plurality of alternating loadings.
In a comparatively overseeable or manageable loading test of, for example, 1000 different load conditions with only 2000 measuring channels or strain measuring points, then at least two million data sets will need to be evaluated, which necessitate a high investment or application of personnel and time for a manual evaluation, in order to derive therefrom an evaluation for the recognition of material fatigue manifestations.
Such an apparatus for the monitoring of the structural fatigue of aircraft and the parts thereof is known from the EP 1 018 641 B1. For that purpose, with the aid of instruments present in the aircraft, the magnitude and the number of turbulence events, the magnitude and the number of the G-loads arising due to flight maneuvers, the number of pressure loading cycles arising in the aircraft, the number of take-off and landing cycles, as well as the number of the wing flap cycles, are detected and stored. These data can be read out by the aircraft or maintenance personnel for the evaluation of fatigue manifestations. However, due to manual evaluation, only a preventive inspection or examination for material fatigue damages can be initiated from the number and the staggered magnitude indications or data, and a defect condition is only then recognizable from the preventive inspection or examination.
From the EP 0 110 865 A2, a measuring apparatus is known for the monitoring of the degree of damage due to material fatigue also on aircraft. For that purpose, several sensors are arranged on the critical locations of the aircraft and the like to be monitored, which sensors detect the material loading and then supply these signals respectively through a separate amplifier channel of a sample and hold circuit and a common multiplexer circuit. An analog-digital converter is connected at the output of the multiplexer circuit, and the analog-digital converter supplies the signals to a comparator, which compares the signals with extreme values stored in a buffer memory. By means of a counter or a summing memory of an evaluating circuit, especially from the extreme values of the signals, a cumulative loading signal is formed, stored and therefrom a degree of damage is calculated. This determined degree of damage is then continuously compared with a prescribed acceptable or permissible degree of damage, and upon exceeding the same is signaled or indicated. This method is, however, only suitable for monitoring the degree of damage of an aircraft that is in operation, for which a prescribable permissible degree of damage must previously have been determined, which is only determinable by complex or costly continuous loading tests on comparable aircraft parts. For that purpose, on the basis of such a continuous loading test, a Woehler stress-cycle diagram with damage lines is produced, albeit manually, from which corresponding limit load cycle criteria depending on the loading magnitude are recognizable, and which serve for the determination of the permissible degree of damage.